Bidirectional control surfaces for use with high speed vehicles, and associated systems and methods

ABSTRACT

Vehicles with bidirectional control surfaces and associated systems and methods are disclosed. In a particular embodiment, a rocket can include a plurality of bidirectional control surfaces positioned toward an aft portion of the rocket. In this embodiment, the bidirectional control surfaces can be operable to control the orientation and/or flight path of the rocket during both ascent, in a nose-first orientation, and descent, in a tail-first orientation for, e.g., a tail-down landing.

CROSS-REFERENCE TO RELATED APPLICATION(S) INCORPORATED BY REFERENCE

The present application is a divisional of U.S. patent application Ser. No. 12/712,083, filed Feb. 24, 2010, and entitled BIDIRECTIONAL CONTROL SURFACES FOR USE WITH HIGH SPEED VEHICLES, AND ASSOCIATED SYSTEMS AND METHODS, which claims priority to U.S. Provisional Patent Application No. 61/187,268, filed Jun. 15, 2009, and entitled “BIDIRECTIONAL CONTROL SURFACES FOR USE WITH HIGH SPEED VEHICLES, AND ASSOCIATED SYSTEMS AND METHODS,” and which also claims priority to U.S. Provisional Patent Application No. 61/155,115, filed Feb. 24, 2009, and entitled “ROCKETS WITH DEPLOYABLE FLARE SURFACES, AND ASSOCIATED SYSTEMS AND METHODS,” which are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure is directed generally to control surfaces for use with high speed vehicles, and associated systems and methods.

BACKGROUND

Rocket powered launch vehicles have been used for many years to carry humans and other payloads into space. Rockets delivered the first humans to the moon, and have launched many satellites into earth orbit, unmanned space probes, and supplies and personnel to the orbiting international space station.

Despite the rapid advances in manned and unmanned space flight, delivering astronauts, satellites, and other payloads to space continues to be an expensive proposition. One reason for this is that most conventional launch vehicles are only used once, and hence are referred to as “expendable launch vehicles” or “ELVs.” The advantages of reusable launch vehicles (RLVs) include the potential of providing low cost access to space.

Although NASA's space shuttle is largely reusable, reconditioning the reusable components is a costly and time consuming process that requires extensive ground based infrastructure. Moreover, the additional shuttle systems required for reentry and landing reduce the payload capability of the shuttle. As commercial pressures increase, the need remains for lower-cost access to space. Aspects of the present disclosure are directed to addressing this challenge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side elevation view of a representative vehicle having bidirectional control surfaces configured in accordance with an embodiment of the disclosure, and FIG. 1B is an aft end view of the vehicle of FIG. 1A.

FIGS. 2A-2C are a plan view, inboard end view, and an outboard end view, respectively, of a bidirectional control surface configured in accordance with an embodiment of the disclosure.

FIGS. 3A and 3B illustrate a representative vehicle during ascent and descent, respectively, in accordance with an embodiment of the disclosure.

FIGS. 4A-4D are plan views of bidirectional control surfaces configured in accordance with other embodiments of the disclosure.

DETAILED DESCRIPTION

The present disclosure is directed generally to bidirectional control surfaces for use with rockets and other vehicles that can fly in both nose-first and tail-first orientations. Several details describing structures and processes that are well-known and often associated with rockets and aerodynamic control surfaces are not set forth in the following description to avoid unnecessarily obscuring embodiments of the disclosure. Moreover, although the following disclosure sets forth several embodiments, several other embodiments can have different configurations, arrangements, and/or components than those described in this section. In particular, other embodiments may have additional elements, and/or may lack one or more of the elements described below with reference to FIGS. 1A-4D.

FIG. 1A is a partially schematic, side elevation view of a vehicle 100 having a plurality of bidirectional fins 150 configured in accordance with an embodiment of the disclosure. FIG. 1B is an aft end view of the vehicle 100 shown in FIG. 1A. Referring to FIGS. 1A and 1B together, the vehicle 100 can be a rocket (e.g., an orbital or suborbital vehicle) that includes a booster or propulsion module 110 carrying a payload module 130. In one embodiment, for example, the vehicle 100 can be a reusable launch vehicle that takes advantage of the ability to fly in both a nose first and tail first direction to recover the vehicle 100 in a vertical, tail first landing. In a particular embodiment, the payload module 130 can be configured to carry cargo and/or crew. In the illustrated embodiment, the payload module 130 has a hemispherical shape. In other embodiments, however, the payload module 130 can have other shapes. In still further embodiments, the propulsion module 110 can be configured to carry additional rocket stages, such as an upper stage.

The propulsion module 110 can include one or more engines having corresponding exhaust nozzles 111 positioned toward an aft portion 101 of the vehicle 100. In a particular embodiment, the vehicle 100 includes five engines, each having a corresponding engine exhaust nozzle 111. The engines are used during the boost phase to propel the vehicle 100 upwardly during ascent. Optionally, some or all of the engine nozzles 111 can pivot to provide thrust vectoring to steer the vehicle 100 during ascent, either alone or in combination with other control systems including other aerodynamic control systems.

The vehicle 100 can additionally include a deployable aerodynamic surface or surfaces, such as a deployable flare 140, positioned toward a forward portion 102 of the vehicle 100. The deployable flare 140 can be stowed during ascent and deployed during descent to stabilize and/or slow the vehicle 100 during a tail down descent and landing. In various embodiments, the vehicle 100 can include deployable flare systems as described in U.S. Provisional Patent Application No. 61/155,115, filed Feb. 24, 2009, and entitled “ROCKETS WITH DEPLOYABLE FLARE SURFACES, AND ASSOCIATED SYSTEMS AND METHODS;” and U.S. Non-provisional Patent Application No. [Attorney Docket No. 34563.8008.US01], filed Feb. 24, 2010, and entitled “LAUNCH VEHICLES WITH FIXED AND DEPLOYABLE DECELERATION SURFACES, AND/OR SHAPED FUEL TANKS, AND ASSOCIATED SYSTEMS AND METHODS,” both of which are incorporated herein in their entireties by reference. In the illustrated embodiment, the vehicle 100 can further include a deployable landing gear 120 (showed stowed in FIGS. 1A and 1B) positioned to allow the vehicle 100 to land in a tail first or tail down orientation.

Although only illustrative of particular embodiments, the propulsion module 110 can have a length L of from about 10 feet to about 50 feet, such as from about 20 feet to about 40 feet, or about 33 feet. The propulsion module 110 can also include a cylindrical or circular cross-section having a diameter D of from about five feet to about 20 feet, or from about eight feet to about 15 feet, or about 13 feet. In other embodiments, the vehicle 100 can have other shapes, sizes and overall dimensions without departing from the present disclosure.

In a particular embodiment, the aft portion 101 of the vehicle 100 includes an aft surface 170. In the illustrated embodiment, the aft surface 170 includes a base region 172 in the proximity of the nozzles 111, and a transition region 174. The transition region 174 transitions between the base region 172 and an exterior surface 103 of the propulsion module 110. In a particular embodiment, the base region 172 can be flat, or at least generally flat, and the transition region 174 can be curved. For example, in a particular embodiment the transition region 174 can have a radius of from about 20 inches to about 50 inches, or about 40 inches. In other embodiments, the base region 172 and/or the transition region 174 can have other shapes, sizes, and/or dimensions.

In one aspect of this embodiment, the bidirectional fins 150 are positioned toward the aft portion 101 of the propulsion module 110. In the illustrated embodiment, the vehicle 100 includes four fins 150 equally spaced around the propulsion module 110, and each of the fins 150 is substantially identical, or at least generally similar in structure and function. In other embodiments, however, the vehicle 100 can include more or fewer fins positioned at different locations around the propulsion module 110, and one or more of the fins can be different in structure and/or function.

As described in greater detail below, the bidirectional fins 150 can be used for vehicle guidance and control during both ascent in a nose-first direction or orientation, and descent in a tail-first direction. In this regard, the fins 150 can be operationally coupled to a control system 162. The control system 162 can include one or more processors, circuits, and/or mechanisms configured to rotate or pivot the fins back and forth about a pivot axis or hinge line 160 in response to control signals received from an on-board guidance system, a remote guidance system, and/or computer-readable media. As described in greater detail below, the bidirectional fins 150 can pivot together in the same direction, at the same rate, and/or to the same angle of attack (“α”); or independently (e.g., differentially) with respect to each other in different directions, rates, and/or different angles of attack, as required to provide the desired vehicle trajectory during ascent and/or descent. In a particular embodiment, the fins 150 can operate between angles of +/−30 degrees. In other embodiments, the fins 150 can pivot to other angles. Further aspects of the fins 150 are described in greater detail below.

FIG. 2A is a planform or side elevation view of the fin 150 configured in accordance with an embodiment of the disclosure. FIG. 2B is an inboard end view of the fin 150, and FIG. 2C is an outboard end view of the fin 150. Referring to FIGS. 2A-2C together, the fin 150 includes a tip 254 spaced apart from a root 252. In one aspect of this embodiment, the fin 150 has a relatively low aspect ratio (“AR”). For example, the fin 150 can have a span S of from about 15 inches to about 45 inches, or about 30 inches. The root 252 can have a root chord RC of from about 60 inches to about 110 inches, or about 83 inches, and the tip 254 can have a tip chord TC of from about 10 inches to about 30 inches, or about 20 inches. As those of ordinary skill in the art will appreciate, the foregoing dimensions are merely representative of certain embodiments of the disclosure. The present disclosure is not limited to these dimensions, and other embodiments can have other dimensions without departing from the present disclosure.

In another aspect of this embodiment, the fin 150 includes a first or forward edge 256 having relatively little sweep, or no sweep, as defined by a first sweep angle A1 of from about 85 degrees to about 95 degrees, or about 90 degrees. The fin 150 can further include a second or aft edge 258 having a relatively high sweep as defined by a second sweep angle A2 of from about 15 degrees to about 40 degrees, or about 29 degrees. In other embodiments, the forward edge 256 and/or the aft edge 258 can have other sweep angles. As used herein, in this particular embodiment the term “forward edge” refers to the edge positioned toward the forward portion 102 of the vehicle, and the term “aft edge” refers to the edge positioned toward the aft portion 101 of the vehicle.

In a particular embodiment, the fin 150 has a symmetrical, or an at least approximately symmetrical airfoil cross-section. More specifically, in the illustrated embodiment the fin 150 has a flat-sided, diamond-shaped cross-section in which the root 252 has a maximum thickness Rt occurring at, or at least proximate to, a midpoint MR of the root chord RC. Similarly, the tip 254 has a maximum thickness Tt occurring at, or at least proximate to, the midpoint MT of the tip chord TC. In a particular embodiment, the maximum thickness Rt at the root chord RC can be from about 6 inches to about 13 inches, or about 9 inches, and the maximum thickness Tt at the tip chord TC can be from about 1 inch to about 4 inches, or about 2.2 inches. In other embodiments, the fin 150 can have other symmetric or non-symmetric cross-sections, as well as other maximum chord thicknesses at the root and/or the tip.

As shown in FIG. 2A, the hinge line 160 is positioned between the midpoint MR of the root 252 and the forward edge 256, and is offset from the midpoint a distance HL. In the illustrated embodiment, the distance HL can be from about 3 inches to about 18 inches, or about 8 inches. In other embodiments, the hinge line 160 can have other positions relative to the forward edge 256, the aft edge 258, and/or the midpoint MR of the root 252.

In a further aspect of this embodiment, the forward edge 256 can have a radius LEr of from about 0.1 inch to about 1 inch, or about 0.25 inch, and the aft edge 258 can have a radius TEr of from about 0.1 inch to about 1 inch, or about 0.25 inch. In addition, the tip 254 can have a radius Tr of from about 0.1 inch to about 2 inches, or from about 1 inch at the midpoint MT to about 0.25 inch at the forward edge 256 and about 0.25 inch at the aft edge 258. Making the tip portion of the fin 150 rounded instead of flat can provide gentler stall characteristics. In other embodiments, however, the forward edge 256, the aft edge 258, and/or the tip 254 can have other shapes, sizes, radiuses and/or other dimensions. For example, in a particular embodiment the tip 254 can be flat or at least approximately flat.

In particular embodiments, the fin 150 can be manufactured from suitable materials known in the art, including, for example, suitable metallic materials such as aluminum, titanium, and/or steel. In other embodiments, the fins 150 and/or portions thereof can be manufactured from suitable composite materials, including graphite/epoxy materials and/or other suitable fiber-reinforced resin materials. Such composite structures can include, for example, composite sandwich structures having a suitable core material covered by a laminated facesheet of composite laminates. In further embodiments, the outer surfaces of all or a portion of the fins 150 can include suitable layers and/or coatings (e.g., ablative coatings) for dealing with the potentially high temperatures experienced during ascent and/or descent of the vehicle 100 (FIG. 1A).

As discussed above, the fin 150 can be implemented to provide guidance and control on a vehicle (e.g., a rocket) that flies in a first direction (e.g., nose first or forward) during ascent and a second direction (e.g., tail first or aft-first) during descent. One feature of the fin 150 is that when the vehicle is flying in an ascent direction, as indicated by arrow A, the fin 150 provides a relatively high change in lift force as the angle of attack (α) of the fin 150 changes. Put another way, the fin 150 demonstrates a relatively high lift slope during ascent, with lift stall occurring at an angle of attack α of from about 8 degrees to about 13 degrees, or at about 10 degrees or more. As used herein, the term “lift slope” refers to the slope of a curve describing the lift, or more specifically the coefficient of lift C_(L), of the fin 150 as a function of angle of attack, α. When the vehicle is flying in a descent direction, however, as indicated by arrow D, the fin 150 demonstrates a relatively low lift slope with a peak lift coefficient C_(L) of at least about 1. Moreover, during descent the fin 150 of this embodiment stalls at angles of attack α greater than about 12 degrees to about 18 degrees, or greater than about 15 degrees. Accordingly, for reasons discussed in more detail below, in the illustrated embodiment the fins 150 are configured to provide a relatively aggressive lift curve during ascent in a nose first direction, and a relatively gradual lift curve, with a relatively high lift peak, during descent in a tail first direction.

In another aspect of the illustrated embodiment, the fin 150 maintains a center of pressure location during all phases of flight that is relatively close to the actuator hinge line 160. This minimizes or at least reduces the torques required to pivot the fin 150 relative to its neutral state and achieve the desired angles of attack. A further aspect of the fin 150 is that it is configured to operate in a flight regime or envelope including both subsonic and supersonic flight, including supersonic flight at a mach number of about four.

As mentioned above, in a particular embodiment the fin 150 can have a symmetrical, or an at least approximately symmetrical airfoil shape (e.g., a diamond-shape or a “double wedge” supersonic airfoil shape). A symmetric airfoil can facilitate predictable behavior during bidirectional flight, and results in the maximum thickness Rt of the root 252 being positioned relatively close to the pivot axis or hinge line 160.

During ascent in the direction of arrow A, the forward edge 256 is the “leading edge” and the planform of the fin 150 represents a relatively low aspect ratio AR lifting surface having a non-swept (or very low sweep) leading edge. In this particular embodiment, this planform creates a moderate to high lift curve slope with stall occurring beyond a desired angle of attack, such as about 10 degrees. During descent in the direction of the arrow D, the aft edge 258 becomes the “leading edge,” and the planform represents a relatively low aspect ratio AR lifting surface having a leading edge that is highly swept at an angle of, e.g., about 60 degrees relative to the airflow. During descent, this highly swept, low aspect ratio AR planform can provide a relatively low lift curve slope with maximum lift occurring at relatively high angles of attack across the entire flight regime. Moreover, during descent this fin planform can provide a lift stall that occurs at angles of attack of about 20 degrees at subsonic speeds, and at more than about 45 degrees at supersonic speeds. During descent, the maximum coefficient of lift can be at least about 1.0 (for subsonic flight) with peak coefficient of lift values closer to about 1.5 during supersonic flight.

A further aspect of the illustrated fin planform is that during both ascent and descent, the center of pressure location is relatively well bounded throughout the range of angles of attack. This can minimize or at least reduce the torque required to control the fin 150. Moreover, with this fin planform many of the aerodynamic conditions that result in relatively high stresses occur when the center of pressure is very close to the hinge line 160. Although the center of pressure position can, in some embodiments, vary to a greater degree, this is expected to occur during fin maneuvers and/or aerodynamic conditions that result in relatively low stresses.

FIG. 3A is a partially schematic, side elevation view of an embodiment of the vehicle 100 during its ascent, as indicated by arrow A. During the ascent or boost phase, the deployable flare 140 is stowed and is accordingly positioned flat against and/or flush with the external surface 103 of the vehicle 100. Moreover, during the ascent phase the landing gear 120 (FIGS. 1A and 1B) can be stowed.

During boost phase, the fins 150 provide a stabilizing effect as they tend to move the center of pressure aft of the vehicle center of gravity. In certain embodiments, the degree of stabilization provided by the fins 150 can be directly proportional to the curve of the lift slope of the fins and, accordingly, the higher the lift slope the greater the degree of stabilization. In certain embodiments, the magnitude of the lift generated by the fins 150 may not be as important as the slope of the lift curve or the need for the lift curve to remain linear, or at least approximately linear, over the operational angle of attack range. As mentioned above, the fins 150 can also pivot to help actively guide and control the vehicle during ascent.

FIG. 3B illustrates the vehicle 100 during its descent phase, as indicated by arrow D. During descent, the deployable flare 140 can be deployed by, for example, pivoting the flare 140 so that it expands outwardly from the external surface 103. As discussed above, this configuration is expected to slow and help stabilize the vehicle 100 during descent. For example, by deploying the flare 140 the center of pressure acting on the vehicle 100 can shift upwardly (e.g., above the vehicle center of gravity) so that gravitational forces acting on the vehicle 100 tend to stabilize perturbations that may be caused by aerodynamic forces acting on the vehicle 100.

During descent of the vehicle 100, the engines are off and no longer thrusting in most, if not all embodiments. In certain embodiments, the engines will remain off and non-thrusting until just prior to touch down of the vehicle 100 in a tail-first orientation at the landing site. As a result, the fins 150 are the dominant aerodynamic control surfaces and the only means, or at least the predominant means, for steering the vehicle 100 during descent.

During descent, the fins 150 are positioned towards the direction of flight and can thus destabilize the vehicle. In certain embodiments, however, having a relatively gentle lift curve can minimize, or at least reduce, the aerodynamic destabilization effect of the fins 150 during descent. However, because the fins 150 are used for vehicle guidance and control during descent, it is also desirable for the fins 150 to be able to provide sufficiently high levels of peak lift. This peak lift will enable the fins 150 to orient the vehicle to relatively large angles of attack when needed during descent.

In another aspect of the illustrated embodiment, the fins 150 are located relatively far aft on the vehicle 100. This can maximize, or at least increase, the ability of the fins 150 to stabilize the vehicle 100 during ascent and control the vehicle 100 during descent. As discussed above with reference to FIGS. 1A and 1B, the aft surface 170 of the vehicle 100 can be rounded in the transition region 174 between the relatively flat base region 172 and the external surface 103 of the propulsion module 110. As a result, moving the fins 150 aft produces a slight overhang gap 390 between the inboard tip of the aft edge 258 (FIG. 2A) and the transition region 174 of the aft surface 170. It is expected, however, that the overhang gap 390 will not negatively affect operation of the fins 150 over the flight regime and mission, including both forward travel during ascent and aft travel during descent.

During descent, the fins 150 are positioned sufficiently behind a bow shock 380. The relatively flat base region 172 of the aft surface 170 tends to move the bow shock 380 outwardly in front of the aft surface 170 during descent of the vehicle 100. As a result, the fins 150 are positioned generally aft or behind the bow shock 380, which can avoid or at least reduce shocks and other high loads on the fins 150 during descent.

There are various aspects of the fin design that are expected to provide favorable characteristics for use with a reusable launch vehicle that can ascend in a nose-first direction and descend in a tail-first direction. For example, the fins 150 are relatively small and, as a result, remain positioned behind the bow shock 380 during both descent and ascent. As discussed above, this can prevent or at least reduce the likelihood that shocks will directly impinge on the fin surface and create high local loads or unsteady, buffeting loads during flight. The relatively short fin span S (FIG. 2A) also facilitates working around the vehicle and performing ground maneuvers such as vehicle lifting, rotation, and/or transportation with conventional on-site equipment.

FIGS. 4A-4D are a series of side elevation views of portions of launch vehicles 400 a-d having bidirectional control surfaces or fins 450 a-d configured in accordance with other embodiments of the disclosure. Referring first to FIG. 4A, the fin 450 a is at least generally similar in structure and function to the fin 150 described in detail above. However, in the illustrated embodiment the fin 450 a includes an aft edge 458 having a non-swept inboard portion 458 a-1 and a highly swept outboard portion 458 a-2 (e.g., an outer one-half portion). In one aspect of this embodiment, having the aft edge 458 with a straight inboard portion 458 a-1 and a highly swept outboard portion 458 a-2 may result in a fin with earlier stall characteristics than the fin 150 described in detail above.

FIG. 4B illustrates a fin 450 b having a relatively low or moderately swept forward edge 456 b and a relatively highly swept aft edge 458 b. Referring next to FIG. 4C, the fin 450 c has a symmetrical, or an at least approximately symmetrical planform in which both a forward edge 456 c and an aft edge 458 c are moderately to highly swept. Referring next to FIG. 4D, in this embodiment the fin 450 d has a highly swept forward edge 456 d and a non-swept or relatively low sweep aft edge 458 d. In this particular embodiment, however, the fin 450 d can rotate a full 360 degrees about a hinge line 460 d to that the planform can be optimized for the direction of flight. For example, in a particular embodiment the fin 450 d can be oriented as shown by the solid line in FIG. 4D for ascent, and then rotated 180 degrees about the hinge line 460 d to the position shown by the dotted line in FIG. 4D for descent. Although the fins illustrated in FIGS. 4A and 4D can have symmetrical cross-sections (e.g., diamond-shaped cross-sections), in other embodiments these fin configurations and variations thereof can have non-symmetrical cross-sections.

From the foregoing, it will be appreciated that specific embodiments of the disclosure have been described herein for purposes of illustration, but that the disclosure may include other embodiments as well. For example, the bidirectional control surfaces 150 can have other shapes and/or arrangements that are different than those shown and described above depending on the type of rocket, mission, etc. Certain aspects of the disclosure described in the context of particular embodiments may be combined or eliminated in other embodiments. Further, while advantages associated with certain embodiments have been described in the context of those embodiments, other embodiments may also exhibit such advantages and not all embodiments need necessarily exhibit such advantages to fall within the scope of the invention. Accordingly, the invention is not limited, except as by the appended claims. 

1-15. (canceled)
 16. A method for controlling an airborne vehicle having a first end portion and a second end portion, the method comprising: aerodynamically controlling the vehicle with one or more control surfaces when the vehicle is flying in a first orientation in which the first end portion leads the second end portion; and aerodynamically controlling the vehicle with the one or more control surfaces when the vehicle is flying in a second orientation in which the second end portion leads the first end portion.
 17. The method of claim 16 wherein the airborne vehicle is a rocket, and wherein aerodynamically controlling the rocket when the rocket is flying in the first orientation includes pivoting at least one of the control surfaces about a hinge line, and wherein aerodynamically controlling the rocket when the rocket is flying in the second orientation includes pivoting the at least one control surface about the hinge line.
 18. The method of claim 16 wherein the airborne vehicle is a rocket, and wherein aerodynamically controlling the rocket when the rocket is flying in the first orientation includes aerodynamically controlling the rocket during ascent, and aerodynamically controlling the rocket when the rocket is flying in the second orientation includes aerodynamically controlling the rocket during descent.
 19. The method of claim 16, further comprising: operably coupling a rocket engine toward the second end portion; and operably coupling the one or more control surfaces toward the second end portion.
 20. The method of claim 16 wherein the one or more control surfaces include a plurality of fins, and wherein the method further comprises: operably coupling at least one rocket engine toward the second end portion; and pivotally coupling the plurality of fins toward the second end portion, wherein aerodynamically controlling the vehicle when the vehicle is flying in the first orientation includes pivoting at least one of the fins about a hinge line, and wherein aerodynamically controlling the vehicle when the vehicle is flying in the second orientation includes pivoting the at least one control surface about the hinge line.
 21. A method of operating a rocket have a forward end portion and one or more rocket engines positioned toward an aft end portion, the method comprising: igniting the one or more rocket engines to launch the rocket from Earth in a first orientation in which the forward end portion leads the aft end portion; aerodynamically controlling the rocket during ascent through Earth's atmosphere in the first orientation by moving a bidirectional control surface; reorienting the rocket from the first orientation to a second orientation in which the aft end portion leads the forward end portion; and aerodynamically controlling the rocket during descent through the Earth's atmosphere in the second orientation by moving the bidirectional control surface.
 22. The method of claim 21 wherein moving a bidirectional control surface includes pivoting a control fin that extends outwardly from the aft end portion of the rocket. 